This section is intended to provide a background or context to the description, and may include concepts that could be pursued but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:
AP access point
ACK acknowledge
CTS clear to send
IEEE Institute of Electrical and Electronics Engineers
MAC medium access control
NAV network allocation vector
PHY physical layer
PLCP physical layer convergence protocol
PPDU PLCP payload data unit
RTS request to send
STA station (not acting as an AP unless otherwise stipulated)
TXOP transmission opportunity
UL uplink (non-AP STA towards AP)
WLAN wireless local area network
In WLAN systems there are potentially multiple stations which may each transmit frames to the access point at any given time. To avoid collisions a signaling protocol has the STA making an announcement before it sends the frame which informs other nodes to keep silent. Specifically, the STA sends a RTS with the length of the data frame or management it wishes to send, and the AP responds with a CTS with the length of the frame that the AP is about to receive. In this manner it is the STA with frame to send which controls transmissions on the channel.
Other nodes in the area of that same AP remain silent upon hearing the RTS so as not to block the CTS that follows, and remain silent for the period of time stipulated in the CTS. Conventionally, this signaling protocol is the WLAN channel access mechanism set forth at IEEE 802.11, and its purpose is to provide protection against interference by ‘hidden terminals’ which are unknown to the AP and STA. This RTS-CTS signaling establishes a NAV protection during which only the STA as holder of the TXOP may decide on the format and type of the traffic to be transmitted.
This general approach is shown at FIG. 1, in which there are five nodes V-Z in the network. Assume Node X is the AP and node W is the non-AP STA that has data or frames to send. The STA/node W sends its RTS with an indication of the length of data or frame or message it has, and awaits a time interval. Within a preset time interval the AP/node X sends a CTS with the indication of length derived from the length indicator value in the RTS. The STA/node W then sends its frame which is acknowledged ACK by the AP/node X. At least node Y is too far to hear transmissions from the STA/node W and so it is a hidden node, but it knows the period of time during which it must refrain from transmitting from the CTS+length message it hears from the AP/node X. There are also specific procedures for imposing a time backoff if there is a collision of RTSs, and for re-transmitting the RTS if the CTS is not received within the interval.
Research is now proceeding toward a new generation of WLAN being standardized as IEEE 802.11ac with amendments to PHY and MAC standards which may increase throughput to 3 Gbit/second in the 5 GHz spectrum and which enables multiple users in a MU-MIMO configuration using up to 80 or 160 MHz bands.
IEEE 802.11ac is to remain compatible with legacy 802.11n user devices. To extend the legacy RTS/CTS scheme directly would result in transmission capacity being wasted in the 802.11ac system. One reason why this would be wasteful is because the legacy devices transmitting in only 20 or 40 MHz bands would, by the RTS/CTS scheme, keep other devices from using the entire 80 or 160 MHz total spectrum in use in the 802.11ac system.